In order to work and to get taxed properly in New Zealand, you need to obtain a tax number from the New Zealand Government. You can apply for this when you arrive in NZ. Don't worry, it isn't as hard as it sounds:
1) Download the Tax Number Application Form (IR595) from the Inland Revenue website (www.ird.govt.nz) For a direct link to the application form click here.
2) Print out and complete the application form you have downloaded. Please note: you need to have a permanent address in New Zealand to get a tax number. If you don't have one already, you can sign up to our BBNZ Mail Service.
3) You now need photocopies of 2 supporting documents such as your passport and international drivers licence - see the form for a list of suitable documents.
4) Now take the completed form and photocopies of your supporting documents to either a New Zealand Post Shop, Automobile Association offices or PickNZ offices.
5) Your IRD number confirmation will be sent to the postal address you supplied within 8-10 days. Alternatively you can ring up to obtain your number. You can then give this to your employer when you start working. For more info on income tax see the NZ Inland Revenue website.